1. Field of the Invention
The invention relates to a plasma display apparatus for displaying an image and a driving method of a plasma display panel.
2. Description of the Related Art
As a thin image display apparatus, a plasma display apparatus with an AC type (alternating current discharge type) plasma display panel has been put into practical use (for example, refer to FIG. 1 of Japanese Patent Kokai No. 2003-302929 (hereinafter referred to as Patent Document 1)). The plasma display panel mounted in the plasma display apparatus comprises: a front substrate on which a plurality of scanning electrodes and sustaining electrodes serving as display lines of a display screen have been formed; and a rear substrate on which a plurality of data electrodes have been formed. The front substrate and the rear substrate are arranged in parallel. A discharge space in which discharge gases consisting of neon, xenon, and the like have been sealed is formed between the substrates. In the discharge space, the data electrodes and both of the scanning electrodes and the sustaining electrodes are arranged so as to cross each other and a discharge cell serving as a pixel is formed at each crossing portion.
To obtain halftone display luminance corresponding to an input video signal, gradation driving using a subfield method is executed to the plasma display panel (for example, refer to FIG. 2 of the Patent Document 1). In the gradation driving, one field is constituted by an initializing period and subsequent eight subfields SF1 to SF8. Each subfield is constituted by a writing period, a sustaining period, and an erasure initializing period. In the writing period in each subfield, a data pulse is selectively applied to each data electrode while a scanning pulse is sequentially applied to each scanning electrode. In this process, a discharge is caused in the discharge cell in the crossing portion of the scanning electrode to which the scanning pulse is applied and the data electrode to which the data pulse is applied. Each discharge cell is set to either a light-emission possible state or a light emission impossible state. In the sustaining period, by applying sustaining pulses to the scanning electrode and the sustaining electrode by the number of times corresponding to a luminance weight of each subfield, only the discharge cells which have been set into the light-emission possible state are allowed to repeatedly perform a sustaining discharge. For example, in the sustaining period of the subfield SF1 having the minimum luminance weight, one sustaining pulse is applied to each of the scanning electrode and the sustaining electrode. In this instance, the sustaining discharge is caused twice between the scanning electrode and the sustaining electrode each time the sustaining pulse is applied. In the subfield SF2 whose luminance weight is larger than that of the subfield SF1 by one stage, since the four sustaining pulses are applied to each of the scanning electrode and the sustaining electrode in the sustaining period, the sustaining discharge is caused eight times.
According to the driving process described above, halftone luminance can be expressed correspondingly to the total number of times of the sustaining discharge caused in the subfields SF1 to SF8. For example, if the sustaining discharge is not caused in all of the subfields SF1 to SF8, a luminance level “0”, that is, the lowest luminance level can be expressed. In the case of expression of an image whose luminance is higher than the lowest luminance level by one step, the sustaining discharge is caused only in the SF1 among the subfields SF1 to SF8. That is, in this case, a luminance level “2” corresponding to the number of times “2” of the sustaining discharge by the subfield SF1 is expressed.
According to the driving method described above, however, since a large luminance difference of “2” exists between the lowest luminance level “0” and the luminance level “2” whose luminance is higher than that by one step, the method has a problem that a luminance change in the image of low luminance cannot be smoothly expressed.